In recent years, in terms of improving productivity, it has been proposed that a robot performs a work which is similar to a work conventionally performed by a human in a manufacturing site, such as a factory. For example, Patent Document 1 discloses a robot which performs works, such as gripping an object, moving the gripped object, and combining a plurality of objects. Patent Document 2 discloses a robot which performs a work to handle a workpiece by using a 3D camera (bin-picking). Patent Document 3 discloses a robot which performs a work to change the way the robot holds a workpiece by using a hand and a contact sensor for the workpiece. Moreover, Patent Document 4 discloses a robot which performs an assembly work (fitting etc.) of a flexible object by using a camera and a force sensor.